1 Answer
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Your Code behind won't be executed anymore once the page / component is in cache.
You could point to the same url with a different querystring (eg : ?lg=fr) and have this query string parameter in your output cache declaration

<%@ OutputCache Duration="1800" VaryByParam="lg" %>

Alternatively, it's often better to rely on the output cache profile / framework of SharePoint, within it you'll also be able to adapt your varbyparam / varybycustom without implementing it on all your user control / webparts / pages.